JPH033747B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing high-purity lithium carbonate. More specifically, the present invention relates to a method for producing high-purity lithium carbonate, particularly with extremely low silicon and anion impurities, using crude lithium carbonate as a raw material. High-purity lithium carbonate is a ferroelectric material such as lithium tantalate and lithium niobate, which is a component of optoelectronic devices such as surface acoustic wave devices, optical modulators, optical switches, and optical waveguides used in TVs, VTRs, etc. in recent years. It grows rapidly as a raw material for single crystals or thin films. However, the lithium carbonate used for such purposes needs to be of much higher purity than general industrial or reagent grade lithium carbonate, and contains alkali metals, alkaline earth metals, transition elements, etc. Of course, it is required that the content of cationic impurities is low, but also that the content of anionic impurities such as silicon, fluorine, chlorine, sulfate, and nitrate is also particularly low. The present invention provides a method for producing high-purity lithium carbonate with extremely low content of silicon and anion impurities, which is suitable for producing surface acoustic wave devices, especially optoelectronic devices where even crystal fluctuation is a problem. (Prior Art) Conventionally, recrystallization methods, reprecipitation methods, ion exchange treatment methods, diaphragm electrolysis methods, and the like are known as methods for obtaining crude lithium compounds, generally lithium carbonate with higher purity than crude lithium carbonate. The recrystallization method is a method in which crude lithium carbonate is dissolved in water to a saturation concentration, water is removed by operations such as heating and reduced pressure, and lithium carbonate is precipitated, thereby removing impurities in the crude lithium carbonate and obtaining high purity lithium carbonate. It is. Also, JP-A-57-
Publication No. 95827 discloses that crude lithium carbonate is reacted with formic acid to produce lithium formate, which is purified by repeating dissolution and recrystallization of lithium formate, and purified lithium formate is treated with carbon dioxide under an ammonia base to produce high-purity carbonate. A method for obtaining lithium is disclosed. However, with these recrystallization methods, small amounts of silicon present in the solution in the form of colloidal silica or silicate cannot be removed by dissolution or filtration.
In addition, due to its low solubility, it easily eutectoids with lithium carbonate during recrystallization, reducing the silicon impurity content.
The disadvantage is that it cannot be reduced to less than about 5 ppm. Furthermore, these methods require repeated operations of crystallization, dissolution, and crystallization, so they require a large amount of steam and heat, are complicated, and have relatively high costs. There is. On the other hand, in the reprecipitation method, crude lithium carbonate and milk of lime are reacted to produce lithium hydroxide, impurities are removed as carbonate along with calcium carbonate precipitate, and then purified lithium hydroxide is reacted with carbon dioxide to produce high-purity lithium hydroxide. A method for obtaining lithium carbonate is known (US Pat. No. 4,207,297). However, this method requires highly purified milk of lime, and quicklime and calcium hydroxide for industrial or reagent use contain various impurities, especially several tens of ppm.
(Hereafter ppm and % are all based on weight.) to number
It usually contains 100ppm of silicon.
The drawback is that refined lithium carbonate is unavoidably contaminated with silicon at a concentration of 10 ppm to several 10 ppm. In addition, the ion exchange treatment method is a method in which an aqueous alkali chloride solution is passed through and brought into contact with a cation exchange resin or a chelate resin to adsorb and remove polyvalent metal cations such as calcium, magnesium, and iron from the aqueous alkali chloride solution. Generally, it is well known as a pretreatment of salt water to be subjected to ion exchange membrane electrolysis (for example, JP-A-56-69220,
JP-A-55-113614, JP-A-54-2998, etc.). However, although these methods are effective in adsorbing and removing polyvalent cations from aqueous alkali chloride solutions, it is well known that silicon impurities such as colloidal silica and silicates that do not exhibit clear cationic properties It is said that the removal of chlorine, sulfate, nitrate, and other anionic impurities is completely insufficient. Therefore, in Japanese Patent Application Laid-open No. 54-43174, lithium carbonate and sulfuric acid are reacted to obtain lithium sulfate, and this is electrolyzed by a diaphragm method, thereby producing high-purity lithium hydroxide on the cathode side and sulfuric acid on the anode side. A method has been proposed in which sulfuric acid is generated and recycled. However, even with this method, a large amount of sulfate anions are present in the anolyte because mineral acids such as sulfuric acid are used to dissolve lithium carbonate, which is poorly water-soluble. However, it is unavoidable that a large amount of sulfate anions are mixed into the lithium hydroxide aqueous solution, and furthermore, lithium carbonate produced from the lithium hydroxide aqueous solution is contaminated by anions. Furthermore, since the liquid on the anode side is a highly corrosive highly concentrated sulfuric acid aqueous solution, the electrolytic cell and equipment require expensive corrosion-resistant members, which is unsatisfactory and unsatisfactory. (Problems to be Solved by the Invention) Under the current situation, the problems to be solved by the present inventors, that is, the purpose of the present invention is to convert crude lithium carbonate into silicon and chlorine suitable for the electronic industry and optoelectronics fields. It is an object of the present invention to provide a new method for producing high-purity lithium carbonate having an extremely low content of anionic impurities such as sulfate radicals and nitrate radicals. (Means for Solving the Problems) The present invention allows poorly water-soluble lithium carbonate to react with carbon dioxide and lithium carbonate slurry without using mineral acids such as sulfuric acid or hydrochloric acid that cause contamination with impurity anions. , lithium carbonate is dissolved into water-soluble lithium hydrogen carbonate, and then the lithium hydrogen carbonate aqueous solution is circulated in the anolyte compartment using an electrolytic cell consisting of two chambers, an anolyte compartment and a catholyte compartment, separated by a cation exchange membrane. On the other hand, a lithium hydroxide aqueous solution is circulated in the catholyte chamber, and lithium hydrogen carbonate is electrolyzed to generate lithium hydroxide in the catholyte chamber.
Then, after removing polyvalent metal ions from the lithium hydroxide aqueous solution if necessary, the above object is achieved by reacting with carbon dioxide to precipitate lithium carbonate powder. The present invention will be explained in detail below. The raw material used in the present invention is preferably low-purity lithium carbonate of industrial or reagent grade, or impurity-containing crude lithium carbonate obtained by primary treatment of ores such as spodumene and lepidolite.
Lithium carbonate is a poorly water-soluble inorganic compound whose solubility in water at 25°C and 80°C is 1.28% and 0.84%, respectively, and it is impractical to purify it as an aqueous lithium carbonate solution due to low productivity per equipment scale. Not. In the present invention, crude lithium carbonate is dispersed in a slurry form in water using a pressure-resistant reaction tank equipped with a stirrer and a gas injection pipe, and reacted with lithium carbonate into which carbon dioxide is blown while thoroughly stirring and mixing to form lithium hydrogen carbonate. It is preferred to dissolve the produced lithium carbonate in water. The concentration of lithium hydrogen carbonate in an aqueous solution increases as the temperature and carbon dioxide pressure increases, but it can dissolve to a concentration of about 10% at room temperature and 1 atmosphere of carbon dioxide.
It is much more soluble in water than lithium carbonate. The dissolution method of the present invention does not use organic acids, mineral acids, milk of lime, etc., but uses carbon dioxide gas, which can be relatively purified, and hydrogen carbonate anions, unlike anions such as mineral acids, can be easily purified. Since it can be converted into carbonate anions, it is a non-contaminating dissolution method with far less contamination from impurities such as silicon and anions than conventional dissolution methods. The concentration of the dissolved lithium hydrogen carbonate aqueous solution in the method of the present invention is 10% or less, preferably 8.5% or less, depending on the stability of lithium hydrogen carbonate at room temperature. The obtained lithium hydrogen carbonate aqueous solution is then subjected to ion exchange membrane electrolysis after insoluble humic substances, carbonates, etc. are separated and removed by filter filtration or the like. The apparatus for electrolyzing an aqueous lithium hydrogen carbonate solution to produce lithium hydroxide according to the method of the present invention comprises an electrolytic cell, a DC generating power source, an attached circulation system, and the like. The circulation system consists of the usual equipment: pipes, pumps, storage tanks, valves, coolers, evolved gas separators, etc. The electrolytic cell consists of two chambers, an anolyte chamber and a catholyte chamber, separated by a cation exchange membrane, using the so-called two-chamber method, with an anode in the anolyte chamber and a cathode in the catholyte chamber.
The number of cells may be one cell or a multi-cell type consisting of a plurality of cells. The anolyte chamber and the catholyte chamber each have an independent liquid circulation system. The cation exchange membrane used in the electrolytic cell has a polystyrene-divinylbenzene copolymer base or a fluorine-based polymer base containing cation exchange groups such as sulfonic acid groups, carboxyl groups, phosphoric acid groups, and phenolic hydroxyl groups. Suitably used. For the anode, a corrosion-resistant electrode made of a corrosion-resistant core material such as titanium or tantalum plated with platinum or ruthenium is used, and for the cathode, for example, nickel or ruthenium is used.
Electrodes such as stainless steel can be used. Electrolysis of lithium hydrogen carbonate is carried out by circulating a lithium hydrogen carbonate aqueous solution to the anolyte compartment and circulating a direct current between the anode and cathode while circulating water or dilute lithium hydroxide aqueous solution to the catholyte compartment. The lithium hydrogen carbonate in the anolyte compartment is electrolyzed, and the lithium ions pass through the cation exchange membrane and move to the catholyte compartment, where they become lithium hydroxide through the reaction (a). (a) Li ⁺ +H ₂ O+e ^- →LiOH+1/2H ₂ ↑ On the other hand, the hydrogen carbonate anion is decomposed into carbon dioxide gas, oxygen gas, and water by the reaction (b) in the anolyte chamber. (b) HCO ₃ ^- →CO ₂ +1/4O ₂ +1/2H ₂ O+e ^-In the present invention, during electrolysis, anions such as chlorine, sulfate, nitrate, and phosphate groups that exist as impurities in the raw material lithium carbonate are Because it is relatively small and has a negative charge, it is attracted to the anode and cannot pass through the cation exchange membrane and remains in the anolyte chamber, so the catholyte chamber contains high-purity lithium hydroxide with almost no anion impurities. is obtained. Furthermore, silicon impurities present in the form of colloidal silica or silicate in the raw material lithium carbonate do not exhibit clear cationic properties, although it is unclear exactly what form they exist in the lithium hydrogen carbonate aqueous solution. Therefore, it hardly passes through the cation exchange membrane and moves to the catholyte compartment, and high-purity lithium hydroxide that is substantially free of silicon impurities can be obtained in the catholyte compartment. In addition, the method of the present invention involves ion exchange membrane electrolysis of a less corrosive lithium hydrogen carbonate aqueous solution with a pH of 7 to 8.5, and the gas generated in the anolyte chamber is carbon dioxide gas, which is less toxic. It has the advantage of less corrosion of equipment and less contamination due to materials, and no special waste gas treatment is required. The appropriate concentration of the lithium hydrogen carbonate aqueous solution in the electrolytic cell is 1 to 8.5%. If the concentration is lower than the above range, a very high voltage is required to perform constant current electrolysis, making electrolysis practically impossible.On the other hand, if the concentration exceeds the above range, lithium hydrogen carbonate decomposes during electrolysis. This is not preferable since there is a possibility that some of the lithium carbonate produced will precipitate in the anolyte chamber. With electrolysis, lithium hydroxide is generated on the catholyte side, and the lithium hydroxide concentration increases. By continuous operation of replenishing water to the catholyte side and taking out the lithium hydroxide aqueous solution so that the lithium hydroxide concentration maintains a predetermined value, or by circulating water or dilute lithium hydroxide aqueous solution to the catholyte side to achieve the desired hydroxide oxidation. A high-purity lithium hydroxide aqueous solution can also be obtained by a batch operation in which the solution is taken out after the lithium concentration has been reached. The concentration of the lithium hydroxide aqueous solution taken out is 2~
It is more preferably 4 to 7% than 10%. Concentration is 2%
If it is lower than 10%, the crystallization yield will be lower when lithium carbonate is precipitated from an aqueous lithium hydroxide solution, and if it exceeds 10%, the diffusion of hydroxyl groups from the catholyte compartment to the anolyte compartment will be greater, resulting in electrolysis. The current efficiency deteriorates when The liquid in the catholyte chamber at the start of electrolysis may be water as described above, but in order to lower the electrolysis voltage at the start, a dilute lithium hydroxide aqueous solution of 0.1% or more is preferable. Appropriate electrolytic voltage during electrolysis is 3 to 15 V, and current density is approximately 1 to 50 A/dm ² . Further, the temperature of the solution circulating in the anolyte compartment and the catholyte compartment during electrolysis must be kept at 40°C or lower, preferably at 30°C or lower. This is because if the liquid temperature exceeds 40°C for a long period of time, the decomposition of lithium bicarbonate in the anolyte chamber will gradually occur, and lithium carbonate will precipitate into the anolyte chamber. Next, the lithium hydroxide aqueous solution obtained by electrolysis is passed through a column filled with a known cation exchange resin or chelate resin to remove polyvalent metal cation impurities such as calcium, magnesium, and iron. Next, using a pressure-resistant crystallization tank equipped with a stirrer and a gas inlet, the lithium hydroxide aqueous solution and carbon dioxide gas are reacted to convert the lithium hydroxide into lithium carbonate and precipitate it. Lithium carbonate is separated and recovered from the obtained lithium carbonate slurry using well-known separation means, washed and dried to obtain purified lithium carbonate. According to the method of the present invention, the content of impurity silicon is 1 ppm or less, more typically 0.5 ppm or less, and the content of anionic impurities such as chlorine, sulfate radicals, and nitrate radicals is each 1 ppm or less. High purity lithium carbonate containing no anionic impurities can be obtained. (Example) Hereinafter, the present invention will be specifically explained with reference to Examples. Example 1 Using a pressure-resistant reaction tank with an internal volume of 120 mm equipped with a stirrer and a carbon dioxide gas blowing tube, crude lithium carbonate containing 80 mm ultrapure water, 530 ppm chlorine, 1350 ppm sulfate radicals, 50 ppm nitrate radicals, and 30 ppm silicon as impurities was prepared. 3.9 kg was charged and dispersed into a slurry. While stirring, carbon dioxide gas was introduced into the reaction tank from the carbon dioxide blowing tube at a pressure of 3 kg/cm ² at a flow rate of 6.5 kg/cm 2 .
The mixture was continuously blown for 3.5 hours at min. to react with lithium carbonate. The pressure inside the reaction tank during the reaction is 0.3
Kg/ ^cm2 , and the pressure at the end of blowing is 2.5Kg/cm2.
rose to cm ² . After the reaction, almost all of the lithium carbonate was dissolved, and an aqueous lithium hydrogen carbonate solution with a concentration of 8.4% was obtained. The lithium hydrogen carbonate aqueous solution was filtered to remove insoluble matter using a 0.2 μm Teflon cartridge filter and transferred to the anolyte storage tank of the electrolyzer. As a result of this operation, no decrease in the content of silicon and anion impurities was observed. A two-chamber electrolytic cell having two chambers, an anolyte chamber and a catholyte chamber separated by a cation exchange membrane, was used as the electrolytic cell. The effective membrane area was 10 ^dm2 . The cation exchange membrane uses Neocepta C66-10F (trade name of Tokuyama Soda Co., Ltd.), which has a sulfonic acid group as a tube functional group and a polystyrene divinylbenzene copolymer as the matrix, and the anode is a titanium plated with platinum, and the cathode is a used sus304. A 0.1% lithium hydroxide aqueous solution was added to the catholyte storage tank. Circulating lithium hydrogen carbonate aqueous solution to the anolyte chamber,
At the same time as the lithium hydroxide aqueous solution was circulated to the catholyte chamber, electrolysis was started at a current density of 15 A/dm ² . The amount of circulation to the electrolytic cell is such that the flow velocity linear velocity in each chamber is 10 cm/sec.
The selection was made so that the internal pressures of both liquid chambers were approximately the same. The temperature of the anolyte and catholyte during electrolysis was kept below 40°C using a cooler. Electrolysis was carried out for 23 hours at an electrolytic cell voltage of 7.1V, and the concentration was 4.9.
An aqueous solution of 39% lithium hydroxide was obtained. The current efficiency was 62%. After passing the obtained lithium hydroxide aqueous solution through a chelate resin containing an iminodiacetic acid group as a functional group, the lithium hydroxide aqueous solution was passed through a 30 crystallization tank.
Lithium carbonate was crystallized by reacting 20 with carbon dioxide. Lithium carbonate was separated using a centrifuge, washed with ultrapure water, and dried in a vacuum dryer at 80°C. The crystallization recovery rate was 71.2%. As a result of analyzing the obtained lithium carbonate, it was found that silicon 0.4ppm, chlorine,
Sulfate and nitrate roots were each less than 1 ppm. Examples 2 and 8 In Example 1, a membrane with a sulfonic acid group and a fluorine-based polymer matrix (Nafion 324: trade name of Dupont Co., Ltd.) was used as a cation exchange membrane; A membrane with sulfonic acid groups on one side and carboxyl groups on the other (nafion)
Crude lithium carbonate was purified in the same manner as in Example 1, except that 901 (trade name of DuPont) was used.
Table 1 shows the experimental results of current efficiency and impurity content in purified lithium carbonate.

[Table] Example 4 In Example 1, 215 ppm of silicon was added as an impurity,
Example 1 except that crude lithium carbonate containing 810 ppm chlorine, 2600 ppm sulfate radicals, and 200 ppm nitrate radicals was used.
Crude lithium carbonate was purified in the same manner. The current efficiency was 61%, silicon content in purified lithium carbonate was 0.7 ppm, and chlorine, sulfate radicals, and nitrate radicals were each less than 1 ppm. Example 5 In Example 1, placing the catholyte in the storage tank
Crude lithium carbonate was purified in the same manner as in Example 1, except that the amount of 0.1% lithium hydroxide aqueous solution was changed to 15, and 23.4% of a 7.1% lithium hydroxide aqueous solution was obtained. The current efficiency was 54%. The impurity content in purified lithium carbonate is 0.5ppm of silicon, chlorine,
Sulfate and nitrate roots were each less than 1 ppm. Example 6 Crude lithium carbonate was purified in the same manner as in Example 1 except that the current density was 30 A/dm ² and the electrolysis time was 13 hours to obtain a 4.9% lithium hydroxide aqueous solution 41. . Current efficiency is 58% and electrolyzer voltage is 12.3V
It was hot. The impurity content in the purified lithium carbonate was 0.4 ppm for silicon, and 1 PPm or less for each of chlorine sulfate and nitrate groups. (Effects of the Invention) As is clear from the above, the present invention has the following effects. By dissolving crude lithium carbonate in a non-polluting manner using carbon dioxide gas and subjecting lithium hydrogen carbonate to ion exchange membrane electrolysis, it is possible to remove silicon and anion impurities, which were completely difficult with conventional purification methods. High purity lithium carbonate containing 1 ppm or less of silicon and 1 ppm or less of anion impurities, which is suitable for industrial and optoelectronic fields, can be obtained. The method of the present invention uses ion exchange membrane electrolysis of lithium hydrogen carbonate, which is less corrosive, and the gases generated in the anolyte chamber are carbon dioxide gas and oxygen gas, which are less toxic and corrosive, so the equipment is expensive. There is no need to use corrosion-resistant materials, and there is no need for special waste gas removal equipment, which reduces equipment costs and is highly economical.

Claims (1)

[Claims] 1. In a slurry consisting of crude lithium carbonate and water,
A lithium hydrogen carbonate aqueous solution obtained by reacting carbon dioxide is used in an electrolytic cell having an anode chamber and a cathode chamber separated by a cation exchange membrane. Electrolysis is performed while passing a lithium hydroxide aqueous solution,
A method for producing high-purity lithium carbonate, which comprises taking out the aqueous lithium hydroxide solution produced in the cathode chamber and reacting it with carbon dioxide to precipitate lithium carbonate powder. 2 The concentration of lithium hydrogen carbonate aqueous solution in the anode chamber is 10
The method for producing high-purity lithium carbonate according to claim 1, characterized in that the amount of lithium carbonate is less than or equal to % by weight. 3. The method for producing high-purity lithium carbonate according to claim 1, wherein the concentration of the lithium hydroxide aqueous solution taken out from the cathode chamber is 2 to 10% by weight.